Significant changes in the local magnetic field
marked the resumption of eruptive activity at Stromboli
volcano on February 27, 2007. After differential magnetic
fields were obtained by filtering out external noise using
adaptive filters and seasonal thermal noise using temperature
data, we identified step-like changes of 1–4 nT
coincident with the opening of eruptive fissures in the
upper part of the Sciara del Fuoco. The magnetic variations
detected at two stations are closely related to the propagation
of a shallow NE–SW magmatic intrusion extending
beyond the summit craters area. These observations are
consistent with those calculated using piezomagnetic
models in which stress-induced changes in rock magnetization
are produced by the magmatic intrusion. No
significant magnetic changes were observed when the first
fractures opened along the NE crater rim. Indeed, the stressinduced
magnetization caused by this magmatic activity is
expected to be too low because of the structural weakness
and/or thermal state of the summit area. The continuous
long-term decay characterizing the post-eruptive magnetic
pattern can be related to a time-dependent relaxation process.
A Maxwell rheology was assumed and the temporal evolution
of the piezomagnetic field was evaluated. This allowed us to estimate the rheological properties of the medium; in
particular, an average viscosity ranging between 1016 and
1017 Pa⋅s was a relaxation time τ of about 38 days.